This invention concerns fluid loading/unloading arms used to transfer petroleum products between two pipelines, one mobile and the other immobile, that are mounted, respectively, on a ship and a quay. More particularly, the invention is concerned with emergency disconnection of such fluid loading/unloading arms.
Fluid loading/unloading arms comprise a fixed and generally upright tubular portion, usually called a riser, adapted to be mounted on a quay and connected to an immobile pipeline, and a maneuverable mobile tubular portion pivotally connected to this fixed tubular portion by a rotary pipe joint, the mobile portion comprising two pipes or tubes connected end-to-end in an articulated manner by rotary pipe joints and ending with a coupling device adapted for connection of the arm to a marine tanker manifold or other mobile pipeline.
Associated with the mobile tubular portion is a static balancing system usually employing a counterweight, adapted to enable the arm, when empty, to remain stable in any configuration. This is necessary for proper execution of the operations to couple or uncouple the arm from the mobile pipeline without having to apply significant force to hold the arm in position.
During loading/unloading operations the ship carrying the mobile pipeline is moored to the quay to which the fixed pipeline leads, and the deformable or maneuverable nature of the arm's mobile portion serves to compensate for low amplitude movements between the ship and the quay that are inevitable when water swells occur.
The arm's mobile portion usually comprises two straight pipe sections (approximately 10 meters long in the case of an arm with an inside diameter of 16 inches) forming an inverted vee, the angle between the sections being variable to facilitate movements with an amplitude of a few meters.
Given the risk of moorings breaking, especially in bad weather, there have already been developed, for safety reasons, emergency disconnection procedures directed to uncoupling the arm from the mobile pipeline before the arm breaks, so that the arm is protected to the greatest possible degree. Any such emergency disconnection requires purging the arm before returning it empty to an equilibrium configuration. Disconnecting the arm when full would inevitably result in uncontrolled movement of the mobile portion totally incompatible with safety requirements in respect of personnel and equipment.
It has therefore been proposed to provide a drain valve at the base of the arm's fixed portion and a "vacuum-break" valve at the apex or top of the inverted vee-shaped tubular portion for venting it to atmosphere. As soon as it is realised that the moorings have broken, the pumps employed to force fluid through the arm are stopped, a flow control valve is shut to isolate the arm from the fixed pipeline, the drain valve is opened, the vacuum-break valve is opened and the liquid contained in the arm is allowed to flow under its own weight, partly into the mobile pipeline and partly through the drain valve into an emergency storage tank. It is only after such gravity draining that disconnection is effected.
Unfortunately, the operation of these valves and the subsequent gravity draining of the arm can sometimes take too long for disconnection to be effected before the arm is damaged. These operations routinely take several minutes which, even if the ship drifts to only a very limited degree, results in an overall arm movement that is outside the arm's envelope, i.e. the arm's permissible range of safe operation. Thus, in many cases this procedure is too slow to protect the arm until disconnection takes place, expecially since there is a significant time lapse between mooring breakage and when such breakage is detected.